1. Field of the Invention
The present invention relates to the field of liquid crystal displaying techniques, and in particular to a method and device of liquid crystal photo-alignment.
2. The Related Arts
As displaying techniques undergo fast development, different types of displays are developed and marketed. The currently available panel display devices include liquid crystal display device (LCD), plasma display panel (PDP), organic light-emitting diode (OLED) display device, and so on. Among these, the liquid crystal display device has the advantages of light weight, small size and low energy consumption, and is widely applied to many fields.
Polymer sustained vertical alignment (PSVA) mode liquid crystal panel is already widely used in liquid crystal display device. The electrode structure of the PSVA mode liquid crystal panel is shown in FIG. 1a. FIG. 1b shows a schematic cross-sectional view along A-A′ line of power-off state of the electrode structure of the PSVA mode liquid crystal panel of FIG. 1a, and FIG. 1c shows a schematic cross-sectional view along A-A′ line of power-on state of the electrode structure of the PSVA mode liquid crystal panel of FIG. 1a. The PSVA mode liquid crystal panel is to add reactive monomer 16 to negative liquid crystal molecules 15 to achieve the photo-alignment of negative liquid crystal molecules 15. In this displaying technique, transparent conductive layers 13, 14 on substrate 11, 12 are neither disposed with bumps nor openings to save the manufacture cost and improve the transmittance of the liquid crystal panel.
In PSVA displaying technique or other displaying techniques requiring photo-alignment, a voltage is applied to the two ends of the liquid crystal cell and UV is applied to excite, the reactive monomer polymerize so as to achieve the photo-alignment of the liquid crystal layer. Throughout the entire process, the UV is uninterruptedly applied to the liquid crystal layer.
According to the polymerization theory, the above reaction process is a UV-excited free-radical polymerization, which includes three steps: chain initiation, chain propagation, and chain termination.
Based on the chemical reaction kinetics, the initiation rate Ri of the chain initiation can be deduced as:Ri=φεI0[M]  (equ. 1-1)Where Ri is the initiation rate of the chain initiation, φ is the photon excitation efficiency, indicating the number of photons required to excite an active free-radical, ε is the molar extinction coefficient of the reactive monomer to UV light, and I0 is the incident light illumination, and [M] is the reactive monomer concentration.
Based on the chemical reaction kinetics, the chain propagation rate Rp can be deduced as:
                    Rp        =                                                            k                p                            ⁡                              (                                                      ϕ                    ⁢                                                                                  ⁢                    ɛ                    ⁢                                                                                  ⁢                                          I                      0                                                                            k                    t                                                  )                                      ⁡                          [              M              ]                                            3            /            2                                              (                              equ            .                                                  ⁢            1                    ⁢                      -                    ⁢          2                )            Where Rp is the chain propagation rate, kp is chain propagation rate constant and kt is the chain termination rate constant.
The chain termination reaction includes coupling termination and disproportionation termination, where coupling termination is the coupling of free-radicals, that is, two active free-radicals are coupled through covalent bond to form inactive saturated molecule; and disproportionation termination is the exchange of electrons or hydrogen atom between two free-radicals, but the two still remain two molecules albeit inactive. These two termination reactions are different, but both reactions between free-radicals.
Because free-radical is a highly active reaction trigger, the photo-triggered free-radical polymerization shows the characteristics of slow initiation, rapid propagation and rapid termination. Based on equ. 1-1 and equ. 1-2, the corresponding kinetic chain length ν can be deduced as:
                    v        =                              Rp            Ri                    =                                                                      k                  p                                                                      (                                          ϕ                      ⁢                                                                                          ⁢                      ɛ                      ⁢                                                                                          ⁢                                              I                        0                                            ⁢                                              k                        t                                                              )                                                        1                    /                    2                                                              ⁡                              [                M                ]                                                    1              /              2                                                          (                              equ            .                                                  ⁢            1                    ⁢                      -                    ⁢          3                )            
The deduction of the above equation is based on a stable system, and without taking the container wall, such as, the substrates on both sides of the liquid crystal, into account.
However, in the known PSVA displaying technique, substrates are placed on both sides of liquid crystal, and the UV light is continuously applied at a constant illumination to perform photo-alignment on the liquid crystal, which results in the insufficient stability and uniformity of the liquid crystal photo-alignment and leading to macromolecular effect and light leakage.
Thus, it is desired to have a method and device of liquid crystal photo-alignment to improve the stability and uniformity of photo-alignment of liquid crystal.